Finding the Source of the Pioneer Anomaly

The team’s eventual geometric model divided the surface of each spacecraft into 3300 patches and incorporated documented thermal properties of the materials used to construct the probes. We used the model to show how heat flowed across and radiated off various parts, and with those results, we estimated the magnitude of the thermal recoil force at different times over the course of the Pioneer missions. After matching the model to the Pioneers’ temperature and electrical readings, we found that the spacecraft did experience a sizable thermal recoil force, corresponding to an excess of about 60 W even after 20 years in deep space. The magnitude of the force was still tiny by Earth standards—about the same as the backward push your car experiences in reaction to the photons spit out by its high-beam headlights. The team found that a good half of the force came from heat from...